Help me understand wheel sizing...my head is spinning...backspacing/offset

The '77 Corvette track car I just bought is currently wearing 15" rims.  15x8 front and 15x10 rear.  Track tires to fit that are nearly non-existent.  My only choice is Hoosier.  I don't want to use those, as I want a more forgiving tire...I'll be having newbies in the car.  Therefore, I want to upgrade to 17 or 18".  However, backspacing and offset are voodoo science to me.  I've tried reading it several times and am still confused.  What I do know is that the '77 uses 5x4.75 bolt pattern and apparently have 4" backspace and -12.7mm offset.  That means nothing to me...I'm so confused.  I know C4-6 rims will fit, but not clear what adapter I would need.  I don't necessarily need 10" rims in the back.  I would think as long as the tires are 245 or wider, I'm in good shape.  Again, this car isn't mean to turn THE fastest lap at HPDE, rather just be fun.  I also don't give a E36 M3 if the wheels are shiny, they're going to get trashed on the track.  I'm just looking for a cheap set of rims to hold the tires I need/want.   It even looks like some S10s and such used 5x4.75 wheels.  If someone put 17" rims on their truck, can I use those (with proper adapters/spacers)???  Help!!!

In reply to SKJSS (formerly Klayfish) :

Backspacing is super easy, it's literally the distance from the hub face to the inner wheel lip. Offset is weird, it's the distance from the centerline of the wheel to the hub face. Positive offset means the hub face is closer to the outside of the wheel than inside and negative offset means the hub face is closer to the inside of the wheel. 0 offset is dead center. 

https://m.tirerack.com/tires/tiretech/techpage.jsp?techid=101&ef_id=CjwKCAjw9qiTBhBbEiwAp-GE0cPizVyWAyiSQt7iG7uTvV07FOblAAPHgnubPcqA0KsctVOWT3ckGxoCrRMQAvD_BwE:G:s&s_kwcid=AL!3756!3!354820921610!b!!g!!wheel%20offset%20chart&gclsrc=aw.ds&gclid=CjwKCAjw9qiTBhBbEiwAp-GE0cPizVyWAyiSQt7iG7uTvV07FOblAAPHgnubPcqA0KsctVOWT3ckGxoCrRMQAvD_BwE

Offset is the measurement of the hub face from the wheel center. Backspace is measured from the outside of the wheel lip to the mounting face.

Backspace gets inaccurate because wheel lips are NOT at all related to the actual tire mounting surfaces. An alloy wheel has a different lip thickness than a steel wheel and different manufacturers use different thicknesses muddying it even further. It is however very easy to measure so that is what most folks use.

Offset without width is a useless number.

Anyway. Go to www.willtheyfit.com and start punching in numbers to see what you like. I know Berk all about old Chevy wheels 

Have you shopped for track tires lately? There's at least a few good choices in 225/15 and 245/15 which it sounds like is exactly what you want. Saves all the issues of dealing with wheel fitment.

dps214 said:

Have you shopped for track tires lately? There's at least a few good choices in 225/15 and 245/15 which it sounds like is exactly what you want. Saves all the issues of dealing with wheel fitment.

The wheels are 15x8 up front and 15x10 in the rear.  Not sure those sizes will fit...especially on the rear.

If the wheel is the right bolt pattern, right width I need, can I get adapters or spacers to make sure it'll fit on the car without rubbing?

Trent said:

Offset without width is a useless number.

That depends on what you're trying to figure out. Backspacing without accurate wheel width is also useless if you're looking at suspension geometry instead of fit.

In reply to SKJSS (formerly Klayfish) :

here's a convenient side-by-side comparison tool:

https://www.wheel-size.com/calc/

SKJSS (formerly Klayfish) said:

dps214 said:

Have you shopped for track tires lately? There's at least a few good choices in 225/15 and 245/15 which it sounds like is exactly what you want. Saves all the issues of dealing with wheel fitment.

The wheels are 15x8 up front and 15x10 in the rear.  Not sure those sizes will fit...especially on the rear.

If the wheel is the right bolt pattern, right width I need, can I get adapters or spacers to make sure it'll fit on the car without rubbing?

225s will fit 8 inch fine. The 245s will fit the 10 inch very slight stretch but wvumtnbkr runs that exact combo in Chump racing. 

Right, a little bit of stretch is fine, honestly even a good thing for most 200TW tires these days that run wide and like a lot of sidewall support.

If you do go up to 17", a set of cheap C4 Corvette wheels and some decent quality spacers are  a good option.

I bet there is a BMW wheel that will fit the bill. Like an E36/E46 17" wheel. 

If you have the width of your wheels it would help immensely. 

SKJSS (formerly Klayfish) said:

The '77 Corvette track car I just bought is currently wearing 15" rims.  

You can't just say a thing like that without at least a picture..

This is it, getting loaded on the trailer yesterday after a fuel pump failure.

This is the one I use, it's not perfect but it is dang close. I save it in my pics and when I come across something I can easily figure it without "the maths".

The confusion sometimes comes from the fact that wheel width is measured INSIDE the lips and backspacing is measured as the total from the mount face to the outside of the back lip.

In truth, offset it doesn't matter where you measure it - inside the lips or outside... as long as you're consistent.  Since the lips are the same, the offset number would be the same.  In fact, that's how I measure it on wheels that aren't stamped or identified.  I lay a straight edge on the back and measure down to the wheel mount face, then lay the straightedge on the front face and measure down through the hub bore to the same mount face.  Subtract and bingo.

LIke that chart points out above.  If you have a 10" wide wheel with zero offset, it would be 5" to both the INSIDES of the front and rear lip, but probably a 5.5" backspace because that's measured to the OUTSIDE of the lip.

The other thing that kinda ticks me off is that nearly all of the pictures that "experts" use to depict backspace and offset are wrong.  Take a look at this one:

Look at the blue and red arrows at the bottom that clearly point to the OUTSIDE of the lips, making you think that 7" wide wheels are measured as the overall width.  This is MSA's own website.  They manufacture wheels and can't accurately describe how wheels are measured?  Nearly all of them do this.

A wheel retailer:

Another wheel retailer:

Fikse is about the only one that accurately shows the real dope.

Here's the thing you probably need to know.  I do this all the time instead of overthinking.

Do your current wheels fit well?  Can you go lock-to-lock on the steering and not rub?  Is there extra space on the inside that you could maybe pull things in a little closer and be OK?

So let's say you're looking at the 15x8 fronts.  Measure the backspacing.  It's probably 4.5" which suggests zero offset.  The wheel mount face is on the centerline of the wheel.  There is 4" of nominal wheel width toward the front to the inside of the lip, and 4" to the back to the inside of the inner lip.  Then 1/2" of lip gives you 4.5" of backspacing.

If everything fits without rubbing and you like what you see, there is nothing wrong with ordering a 17x8 wheel with the same zero offset.  It will fit the same.  You just have more wheel diameter and you get tires with shorter sidewalls.

So you're under there looking and let's say you have plenty of space to the inner fenders, calipers, control arms, and sway bars, so you want to fit more width.  If you have at least 1/2" clearance to everything, you can get a 17x9 wheel with the same zero offset.  That will add 1/2" to the inside and 1/2" to the outside.

Maybe you're under there and you see that stuff is super close to rubbing but you still want something wider.  In that case you can get a 17x9 with a 13mm (about 1/2") negative offset.  So instead of adding a 1/2" in and a 1/2" out, you're adding 0" in and 1" out.

The real reason that offset and backspacing is equally important is for simplicity.  In this last example, you're looking at no spare space toward the inside, you can shop for wheels with the same backspacing and know they'll fit.  Regardless of the width you choose, the backspacing will be the same.

I just figured out what we need in life.... an animated depiction with three lockable sliders.  One of those where you can grab the slider for offset and watch what it does, then lock that slider and grab the one for backspace or width.

Probably not an easy task but someone here could probably come up with some code for it.

Here is an example of what I'm thinking of:  Projection Calculator - Throw Distance and Screen Size (projectorcentral.com)

Curtis73 (Forum Supporter) said:

I just figured out what we need in life.... an animated depiction with three lockable sliders.  One of those where you can grab the slider for offset and watch what it does, then lock that slider and grab the one for backspace or width.

Probably not an easy task but someone here could probably come up with some code for it.

Here is an example of what I'm thinking of:  Projection Calculator - Throw Distance and Screen Size (projectorcentral.com)

The closest thing I know of for that is this site: https://www.wheel-size.com/calc/

In reply to Curtis73 (Forum Supporter) :

I made one of these in a Solidworks sketch, but it only works if you have the software.  While looking how to modify my Fox chassis car I made one for roll center and ackerman, super helpful.

Back on subject my stock 18 x 9.5 +45 wheels fit well but I can fit up to an 11' rim with proper offset and spacers for the front.  I would love an 18 x 10, that is .5 inches wider or 12.7 mm.  If I keep the same offset at +45mm from the center of a 10" wheel instead of a 9.5   This will give me 7mm more backspacing and 5.7mm more poke out of the fender.

WHAT?  The problem lies in offset in mm while backspacing and rim width is in inches.

10" rim =254mm 

Divided by 2 equals rim center line, 127mm 

Add offset of +45  to center line = 172mm = 6.77" backspacing

10" rim minus 6.77" backspacing = 3.23" from hub to fender.

10"rim * 25.4mm/" = 254mm, / 2 = 127mm, +45 = 172mm, /25.4 = 6.77, 10"- 6.77"= 3.23"

9.5" rim same calculations 6.49 BS 3.01 hub to fender.

This shows my 10" rim will poke out .22 inches more than the 9.5 with a backspacing of 6.77" vs 6.49".  If I change my offset to +50 the fit will match what I currently have.  .22 * 25.4 = 5.59 mm.  

This is all fine except an S197 Mustang can handle less offset.  Apex offers an 18x9.5 +35 and an 18x10 +40, these will have 10mm more hub to outer lip dimension than my stock wheels, not hella flush but will fit.  This is for an ideal square setup 18x11 will fit with more offset to fit the rear and then use of spacers up front to clear the strut.

Sorry, too much coffee this morning.

In reply to rslifkin :

That is way easier.

In reply to Curtis73 (Forum Supporter) :

Thank you for the explanation.  It helps a lot...I was kind of looking for the "sizing for dummies" version...cause I'm a dummy!  I'm a visual person, so the diagram helped and your write up let me see it in my head.

For now, to avoid more confusion for myself and expense, I ordered a set of 245/40/15s.  They'll fit what I've currently got.  Probably going to look a bit silly with a small rim size and short sidewall, but they'll work.  I'll continue to make more sense out of this and pick up 17 or 18s, so that when I burn these 15s off, I am ready to upgrade.  I found a set of factory C4 rims that are 17x9.5 for cheap.  Going to try to snag those and then figure out what I'll need to make them fit.

According to Chandlers offset chart you are going from a 4" backspacing to a 7.5" back space, so it will require a 2.5 - 3.0" spacer to bring the tires out to the fender edge.

That is a big spacer, with out it the tires will be very tucked in the fenders and probably hit stuff.

akylekoz said:

In reply to Curtis73 (Forum Supporter) :

I made one of these in a Solidworks sketch, but it only works if you have the software.  While looking how to modify my Fox chassis car I made one for roll center and ackerman, super helpful.

Back on subject my stock 18 x 9.5 +45 wheels fit well but I can fit up to an 11' rim with proper offset and spacers for the front.  I would love an 18 x 10, that is .5 inches wider or 12.7 mm.  If I keep the same offset at +45mm from the center of a 10" wheel instead of a 9.5   This will give me 7mm more backspacing and 5.7mm more poke out of the fender.

WHAT?  The problem lies in offset in mm while backspacing and rim width is in inches.

10" rim =254mm 

Divided by 2 equals rim center line, 127mm 

Add offset of +45  to center line = 172mm = 6.77" backspacing

10" rim minus 6.77" backspacing = 3.23" from hub to fender.

10"rim * 25.4mm/" = 254mm, / 2 = 127mm, +45 = 172mm, /25.4 = 6.77, 10"- 6.77"= 3.23"

9.5" rim same calculations 6.49 BS 3.01 hub to fender.

This shows my 10" rim will poke out .22 inches more than the 9.5 with a backspacing of 6.77" vs 6.49".  If I change my offset to +50 the fit will match what I currently have.  .22 * 25.4 = 5.59 mm.  

This is all fine except an S197 Mustang can handle less offset.  Apex offers an 18x9.5 +35 and an 18x10 +40, these will have 10mm more hub to fender room than my stock wheels, not hella flush but will fit.  This is for an ideal square setup 18x11 will fit with more offset to fit the rear and then use of spacers up front to clear the strut.

Sorry, too much coffee this morning.

 

The problem with this math is that a 10" wide wheel is not actually 10" from edge to edge. It will be roughly 1" wider, and that error is compounded when you use backspacing to get offset and then calculate the outer lip location from that offset.

A faster, easier option is to use offset to figure out the movement of the centerline of the wheel and then add/subtract 0.5" per inch of wheel width.

Back on subject my stock 18 x 9.5 +45 wheels fit well but I can fit up to an 11' rim with proper offset and spacers for the front.  I would love an 18 x 10, that is .5 inches wider or 12.7 mm.  If I keep the same offset at +45mm from the center of a 10" wheel instead of a 9.5   This will give me 7mm more backspacing and 5.7mm more poke out of the fender.

Easy math.

If you keep the same offset and go from 9.5" to 10", you will have 0.25" more backspace and 0.25" more poke. That's because offset is defined by the centerline of the wheel. I'm assuming the width of the actual lip around the edge of the barrel is a constant here. 

If I change my offset to +50 the fit will match what I currently have. 

5mm = .2". If you go from a +45 to a +50, both the inner and outer lips will move in by 0.2". Close enough in this case, a 9.5" wheel with a +45 offset will have nearly the same poke as a 10" wheel with a +50. It will have just over 0.5" more backspace, so you'll have to make sure you have clearance at the strut.

Spacers are simply a way of changing offset easily. They can only lower it, not raise it.

Keep in mind that wheel width will change the effective tire width as well by how pulled the sidewalls are. A tire might clear the fenders on an 8" wheel but not a 9". I won't even get in to the variations between tires of the same size but different makes :)